DEV: Avoid repeating draft key logic in multiple places.

lib/post_creator.rb

@@ -303,7 +303,7 @@ class PostCreator
 
   def draft_key
     @draft_key ||= @opts[:draft_key]
-    @draft_key ||= @topic ? "topic_#{@topic.id}" : "new_topic"
+    @draft_key ||= @topic ? @topic.draft_key : Draft::NEW_TOPIC
   end
 
   def build_post_stats

spec/components/post_creator_spec.rb

@@ -301,9 +301,8 @@ describe PostCreator do
       end
 
       it 'creates post stats' do
-
-        Draft.set(user, 'new_topic', 0, "test")
-        Draft.set(user, 'new_topic', 0, "test1")
+        Draft.set(user, Draft::NEW_TOPIC, 0, "test")
+        Draft.set(user, Draft::NEW_TOPIC, 0, "test1")
 
         begin
           PostCreator.track_post_stats = true